Buy your ticket early.
Get to the departure point early...
...so you can get your choice of seats. The best seats for tall folks (I am 6'1") are the front seats on the second level. However all the seats have more legroom than your ordinary bus.
On the other hand, barring unforeseen delays, the NYC-Phila trip is about 2:15 and won't cripple you for life.
But sit upstairs. It's more fun.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher